#406 SQLite PRAGMA tuning for throughput

[ajianaz]

Problem

SQLite is opened with only journal_mode=WAL; foreign_keys=ON. Missing key throughput PRAGMAs.

Location

src/index/mod.rs:35-36 and src/engine/db_writer.rs:184-185

Solution

Add to open_global_index():

PRAGMA synchronous = NORMAL;
PRAGMA cache_size = -64000;
PRAGMA mmap_size = 268435456;
PRAGMA temp_store = MEMORY;

Expected Impact

1.2-1.5x overall. Trivial change, zero risk.

[ajianaz]

Closed by #421 — N+1 get_symbol_by_id replaced with single WHERE id IN (...) query via batch_get_symbols().